Mozilla plans to drop support for all NPAPI plugins except for Flash in March 2017, when Firefox version 52 is released. See this article and this blog post for details.

Steps to take

If the installed add-on is disabled

Extension or theme

At the top of the Firefox window, click on the Firefox buttonOn the menu bar, click on the Tools menuAt the top of the Firefox window, click on the Tools menu, and then click Add-ons. The Add-ons Manager tab will open.Click the menu button
and choose Add-ons. The Add-ons Manager tab will open.

In the Add-ons Manager tab, select the Extensions panel.

On the "Tools for all add-ons"
menu, make sure that the Update Add-ons Automatically menu item is checked.